Field of the Invention
An apparatus to determine a structure and pathology of an ophthalmic tissue described herein relates to imaging and image processing systems for ophthalmology. More particularly, the embodiments disclosed herein relate to the field of surgical procedures to treat retinal pathologies such as epiretinal membrane, macular holes, and macular pucker.
Description of Related Art
Epiretinal membrane (ERM) is a disease in which a layer of tissue grows on the interior surface of the retina. While there may be multiple causes for this pathology, it usually is a natural aging degeneration. As shown in FIG. 1, a retina 110 is composed of three main tissue layers: an internal limiting membrane (ILM) 111, in contact with vitreous gel 145 in the vitreous cavity 140, a nerve fiber layer (NFL) 112, and the optically sensitive neural layer. Retinal pigment epithelial cells are located (RPE) under the retina 113. Underlying the RPE layer is the choroid 115, which is a tissue containing blood vessels to provide oxygen and metabolic support to the RPE cells. Separating the RPE layer 113 and the choroid 115 is Bruch's membrane, allowing exchange of nutrients from the choroid 115 into the metabolically active RPE cells and waste material from the latter into the former.
As a result of the growth of an epiretinal membrane, the retina may become contracted or wrinkled in the macula area. The retina may become elevated away from the RPE causing damage to retinal function. These deformations result in defects of image formation at the macula, and need to be removed using a vitrectomy surgical procedure.
Vitreomacular traction is another pathological condition of the retina. Excessive adhesion between the vitreous and the ILM may result in the retina being elevated away from the RYE. As vitreous gel 145 moves anteriorly or is contracted, it may tear away portions of the inner surface of the retina into the vitreous chamber.
During surgical procedures to treat the above and other retinal pathologies it is necessary for the surgeon to distinguish between healthy portions of the retina and affected portions. The determination needs to be made in real time, as the surgeon proceeds with the intervention. Furthermore, the determination should require little involvement by the surgeon. The surgeon's attention should be focused on the physical procedure rather than analyzing ancillary information.
State-of-the-art methods to distinguish different tissue types involve the use of fluorescence techniques with differentiated markers. In a fluorescent marker approach, fluorophores emitting different colors of light are combined with suitable carriers that attach to a specific tissue. As a laser or other excitation light scans certain areas, the illumination spot turns into a different color, indicating the type of tissue being illuminated.
Unfortunately, the fluorescence approach may not be used for the treatment of retinal pathologies as described above. Typical fluorescent markers such as indocyanine green (ICG), trypan blue, and other stains have been used to stain ILM, with negative results. ICG is toxic and needs to be administered in low doses, and trypan blue produces weak stains that are difficult to see. Furthermore, there are no stains specific to ERM, and particulate marking of the vitreous humor, ERM, and ILM (e.g. using triamcinolone) is non-specific.
Other commonly used techniques may include tissue selective staining and observation with white light. ICG, Trypan blue, and Membrane Blue, are examples of some of the stains that may be used. The disadvantages of tissue staining are similar to those of fluorescence techniques mentioned above: toxicity to tissue (especially to sub-retinal tissues such as choroid 115) and the need to remove the dye after the procedure. Therefore, there is a need for a method and an apparatus to detect and determine tissue structure on an area to assess whether or not to perform a surgical procedure on that area. Also, a method is needed to detect tissue structure in real time without surgeon intervention to analyze data before making the determination.